A number of reasonably effective methods and related apparatuses exist for charging sealed Ni-CAD and Ni-MH batteries. Fast charging of Ni-CAD and Ni-MH batteries is usually performed with a multiple level current source that is controlled by sophisticated mechanisms. These mechanisms work on various schemes including keying off variations in the temperature as well as methods which employ ratios which work off changes in the voltage with respect to time, such as the ratio -.DELTA.V/.DELTA.t or changes in the temperature such as .DELTA.T/.DELTA.t schemes. The mechanisms are generally implemented through integrated circuits. The cost of these integrated circuits are prohibitive for many applications in consumer products. In addition, most of the available integrated circuits require an external 3 to 5-volt supply. For autonomous dual cell systems, battery voltage can be as low as 2 volts; therefore, a control circuit which overcomes this problem would have to be included.
However, all of these methods and the current apparatuses available have substantial expenses related to their implementation in relation to many of the potential applications for which they can be used. Additionally, many of the devices and circuitry necessary to implement these complex charging schemes lack the compactness needed for their prospective applications.
Thus, the problem of developing a useable method and apparatus which will recharge Ni-CAD and Ni-MH batteries in a relatively quick, efficient and economical manner through use of a compact apparatus has existed for quite some time. Ni-CAD and Ni-MH batteries are generally readily available and have a wide variety of applications. Many applications for their use involve relatively small hand-held electronic devices in which size and cost represent crucial factors in their design. Such devices include hand-held electric razors and similar consumer devices.
Voltage termination of fast-charge current is a potentially less expensive alternative, but charge termination based on voltage sensing is not recommended by battery manufacturers because of the very low resistance of these cells. Charging a battery with a voltage source set too high would result in large currents that would overcharge, damage and perhaps destroy the battery. In addition, Ni-MH and Ni-CAD cells have negative voltage temperature coefficients. In overcharge, the battery heats up and its voltage decreases, making the problem even less manageable. Lead-acid car batteries, vented Ni-CAD and similar batteries used in industrial applications, because of their differing characteristics, are quite amenable to voltage sensing. However, these types of batteries are ill-suited for use in small consumer products.
One attempt to solve many of these problems is illustrated in FIG. 1. Power bipolar transistor (1) in combination with windings (2) and (3) and Schottky diode (4) is used to transfer energy from the input voltage source (5) to the two battery cells (6) and (7). Current sense resistor (8) in combination with biasing-circuitry/DC-offset circuitry (9) and base-drive transistor (10) ensure that the rate of transfer of energy is limited. Base-drive-supply circuit (11) completes the fast rate charger.
The fast rate charge is interrupted through the use of transistor (12) and the voltage dividing elements consisting of trimming resistors (13) dividing resistors (14), diodes (15), (16), base-drive transistor (10) and Schottky diode (4). When biased sufficiently transistor (12) keeps transistor (10) ON thus preventing transfer of energy through transistor (1). The operation of the circuit results in the waveform of FIG. 2.
The above circuit has two very serious deficiencies. First, the battery voltage at which fast charge interruption will occur can vary considerably from one manufactured unit to the next. This is due to the parameter variances of the the multitude of components in the dividing circuits. It also is caused by the effect of the biasing-circuitry/DC-offset circuitry (9) and of the base-drive-supply circuit (11) and the base current required to turn off transistor (1). Second, the circuit resistance between the energy delivering circuit and battery cells (6) and (7) is very low. Since the effect of transistor (12) is to effectively regulate the battery voltage, voltage adjustment must be very accurate in order to avoid overcharging of the battery.
In an attempt to solve these problems, the circuit of FIG. 1 uses an intricate network of trimming resistors (13). However, this scheme requires that 100% of the unit will have to be trimmed. Thus, this solution creates its own significant problems. Accurately determining which resistors to trim is very difficult and may even require a two step trimming procedure. In addition, trimming is perform by manually cutting the tracks, a very undesirable feature for a product which must be mass produced.
The circuit of FIG. 1 also has another undesirable feature, whenever the battery is fully or nearly fully charged, the power supply completely stops for long period of time as shown in FIG. 2. A visual indicator, not shown on FIG. 1, is then turned off. Although the unit is still connected to the voltage source (5), there is no visual indication that the unit is working. It would be desirable to always have an indication that the power supply is functional and also, if possible, that the visual indicator should indicate in some manner the state of charge of the battery when charging.
Thus, a need exists for a fast, efficient and economical way to recharge sealed Ni-CAD and Ni-MH batteries. In particular, one that can adapt the use of a voltage termination and sensing technique and do so in an economical and efficient manner. A method and apparatus that will charge with a voltage sensing method and apparatus Ni-CAD and Ni-MH batteries, while overcoming the three major problems associated with this technique, namely: low resistance between the batteries, negative temperature coefficients of the batteries and voltage source setting.